Wide-band high-frequency transformer



y 4, 1948- D. MACKEY 2,441,116

WIDE -BAND HIGH-FREQUENCY TRANSFORMER 1 1. file/018 T a Filed June 4, 1943 Tlcii.

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I I l I I I I l 1 l f z N: 29 30 3/ Me. FFQQUENCYFZ? 30 323 INVENTOR flax 440 #4542)! ATTORNEY Pat'ented May 4,

WIDE-BAND HIGH-FREQUENCY TRANSFORMER Donald Mackey, mason Heights, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application June 4, 1943, Serial No. 489,663

9 Claims. (Cl. 171-777) This invention relates to wide-band transformers such as may be employed in the radio frequency (R. F.) stages, or in the intermediate frequency (I. F.) amplifier stages of television or other receivers of the superheterodyne type.

It is common practice in superheterodyne receivers for reception, for example, in the broadcast band to use mutually-coupled, magnetic-core transformers as the coupling means between stages. Because of the relatively narrow band of frequencies that need be passed by such transformers they have proved quite satisfactory. However, the use of transformers of similar construction, but designed for higher frequencies such as are encountered in television and pulse reception, has been found to be unsatisfactory for the reason that only a limited range of frequencies can be transmitted thereby.

It is therefore the main object of the present invention to provide a novel core arrangement in an I. F. or R. F. transformer operating at high frequencies whereby the band-width of the transmitted frequency band is materially increased.

It is another object of the invention to secure, together with suflicient coupling, higher gain than heretofore obtained in wide-band. doubletuned I. F. or R. F. transformers as required for television and pulse reception, with a minimum of components.

Still another object of the invention is to increase the coefficient of coupling between the coils of the transformer and to maintain the coupling constant, resulting in a wider frequency band at a given frequency than heretofore obtainable through mutual magnetic coupling alone.

A further object of the invention is to obtain the required band-width by means of mutual magnetic coupling, thereby obtaining a more symmetrical response curve than that obtained by the usual methods of wide-band coupling.

Still a further object is to provide a high frequency coupling device which is constituted by a pair of inductance coils in magnetically coupled relation, a ferro-magnetic core disposed in the field of both said coils for effecting increased coupling between said coils, and a second ferro-magnetic core adjustable relatively to only one of said coils, said coils and said cores being so disposed that movement of said cores relative to their respective coils alters the effective inductance of the respective coils while maintaining the degree of coupling between them substantially constant.

The'novel features characteristic of my invention are set forth with particularity in the appended claims. The invention itself, however,

both as to its organization and mode of operation together with-further objects and advantages thereof, will best be understood by reference to the following description taken in connection with the accompanying drawing in which Fig. 1 is a view in cross-section of the coil and core arrangement utilized in conventional high frequency magnetic core transformers; Fig. 2 shows the response curve for the transformer shown in Fig. 1; Fig. 3 is a view similar to that of Fig. 1 showing the coil and core arrangement of a high frequency transformer according to the present invention; Fig. 4 shows the response curve for the transformer illustrated in Fig. 3; Fig. 5 illustrates schematically a circuit of an I. F. amplifier embodying the present invention, and Fig. 6 is a portion of the circuit of Fig. 5 illustrating schematically a modified form of transformer coupling unit in accordance with the invention.

Referring first to Fig. 1, the conventional magnetic core transformer or coupling unit is seen to comprise a primary winding I and a secondary winding 2 which are coaxially wound and axially spaced on a common insulating tube 3. Magnetic, powdered iron cores 4 and 5 arranged for axial movement within the tube are associated with the respective transformer windings and are adapted to adjust the inductance thereof. Adjusting screws 6 and l at the ends of the cores are provided for that purpose.

In the, conventional transformer, the cores 4, 5 are arranged to work at opposite ends of the respective windings l, 2 so that adjustments in core position do not affect the coupling coeflicient therebetween. Thus the maximum coupling is fixed by the geometry and spacing of the two windings. An intermediate frequency transformer such as described above, but for operation in the broadcast band, is disclosed in Harvey Patent No. 2,283,924 (see Fig. 2).

In Fig. 2 of the'accompanying drawing there is shown the response curve for a transformer having the construction shown in Fig. 1 (but without the coupling impedance X) and operating at 30 megacycles. As indicated the transformer passes the frequencies between 29 and 31 megacycles, or a band-width of 2 mc., with substantially uniform gain.

In some instances, in order to increase the coupling between the transformer windings, coupling impedance X shown by the dotted connection between the high potential ends of the windings has been employed. The impedance may consist of a capacitance or an inductance, and while an increased band-width is obtained there- 3 by, losses are introduced into the circuit and the frequencies above and below the passed band are unequally attenuated resulting in an unsymmetrical response curve.

The high frequency coupling device in accordance with my invention is shown in Fig. 3 wherein the primary and secondary windings 8 and 8, respectively, are placed closer together in. an axial direction on the coil supporting tube 3, and the magnetic core i0, preferably the one that is associated with the secondary winding, is made somewhat longer than the space between windlogs and is positioned in the field 01 both windings. The second core ll is associated with the primary winding 8 in the conventional manner. By virtue of this arrangement the permeability of the path irom primary to secondary due to the core i is increased, thereby realizing an increase in coupling. Since the coupling between the coils involves solely mutual magnetic coupling, a more symmetrical response is obtained, resulting in better attenuation of signals on adjacent channels. Also, the elimination of the coupling impedance X (shown in Fig. 1), besides effecting a saving in the cost of the coupling unit, contributes to the attainment of a higher L. C. ratio oi the double-tuned band-pass circuits.

By making the length oi the coupling core iii shorter than the space between extreme ends of the two windings and longer than the space between adjacent ends of the two windings, the coupling core may be moved back and forth without affecting the permeability of the entire system, and hence without affecting coupling. The desired, degree of coupling may be initially obtained by adjusting either the spacing between windings, or the length of the coupling core ID, or the permeability of the material used in the coupling core, or any combination of these means. Movement of the coupling core in a given direction results in an increase of indu tance of one winding and a decrease in the other. Thus the secondary winding 9 may be tuned by moving the coupling core ill, and the primary winding 8 may be tuned by means of the corn ventionally located core ii. The latter core must provide sufllcient adjustment to compensate for the adjustment position of the coupling core plus the normal variations for which adjustment of inductance is required.

While it is not generally essential that the coils differ in length, it is essential that the coil 8 be sumciently long to accommodate the axial motion oi. hath cores necessary to produce the required degr e of adiustmeht, whereas the coil.

9 need accommodate only the movement of the coupling core iii. It is desirable therefore, there is to he a difference in the winding lengths, to have as the longer winding the winding that is associated with. the core that 'serves only to adjust the inductance. In the disclosed arrangement it is the winding 8. In .eirtreme cases, it may be necessary to deliberately make this winding longer by changing the winding pitch in order to accommodate the re uired adjustment.

' In a practical embodim nt of the coupling transformer constructed in} accordance with my invention, the fiber tube 3 had an inside diameter of .253" and a length of 1%"; the primary winding 8 consisted of a single layer of 22 turns of #30E wire wound to a length of /s4"; the secondary winding 9 was axially spaced from the primary 1; of an inch and consisted of a single layer of 15 turns of #BOE- wirewound to a length of /M"; the magnetic core II was .246"

in diameter and /4" long: and the magnetic core in was .246" in diameter and /3" long.

Fig. 4 shows the response curve obtained with the transformer constructed as shown in Fig. 3 and operating at the same frequency, 30 mega.- cycles, as the transformer 01 Fig. 1. The frequencies that are passed with substantially uniiorm gain extend between approximately 27.7 and 32.2 mc., giving a band width of approximately 4.6 mc. In both cases the geometry of the windings was approximately the same so that the increased band width shown in Fig. 4 is due entirely to the core location.

In an experimental set-up a 30 mc. transformer stage, approximately 4.6 mc. wide, using the transformer 01 Fig. 3, showed a stage gain oi 8.5. In the same set-up a capacity coupled transformer (the high potential ends of the windings connected by a coupling condenser) with the same band width showed a gain of 6.7, representing a ratio or 1.26 to l in favor of the transformer embodying the present invention.

Fig. 5 represents a typical I. amplifier circuit in which the. transformer or coupling unit of Fig. 3 may be employed. The transformer primary 8 shown in this instance to extend over a greater length than the secondary is included in the plate circuit oi an I. F. amplifier tube I2 and the transformer secondary 9 is connected to the signal control grid of a tube i3 which may be a second I. F. amplifier or the conventional second detector utilized in superheterodyne receivers. The magnetic cores H and ill conespond to the similarly numbered cores oi Fig. 3, the former serving to adjust the inductance of the primary and the latter serving to adjust the inductance of the secondary as well as to este lish increased coupling between the two windings. The windings are individually adapted to be tuned by the inherent capacitance of the circult and of the tubes to which they are coupled to :i'orm a band-pass selector for passing the intermediate frequency signals. The tuning capacitances of the windings are indicated by the dotted line condensers l4 and i5 for the reason that these capacitances may be comprised in whole or in part, preferably in whole, of the inherent capacitance of the circuit. The shunting resistance i8 across the primary is used to obtain a hat frequency response, and the resistancc-capaclty combination ll connected to the athoclc of each tube is A conventional self coupling core it to be the co the inductance the secon understood that it may be stead the inductance of t shown in Fig. 6 where to provide the increased on windings and at the sam adjust the inductance of core H. being. associated with only the secondary 9' for adjusting its inductance. In instance the winding 9 may be of a greater length than the primary 8' as shown, since as previously mentioned it is advisable that the core which serves only to adjust the inductance be associated with the longer winding.

While I have shown and described certain preferred embodiments of my invention, it will be understood that various modifications and changes will occur to those skilled in the art without departing from the spirit and scope of this mart] the .the windings, a first axially-adjustable core of comminuted magnetic material disposed within the tube and in overlapping relation with the adjacent end portion of said windings for increasing the degree of coupling between the windings and, upon adjustment thereof, for altering the inductance of one of the windings without substantially effecting a change in the increased degree of coupling between the windings, and a second axially-adjustable core of comminuted magnetic material disposed within the tube and cooperatively related with the other windingv for altering the inductance of said latter winding.

2. A wide-band high-frequency transformer comprising primary and secondary windings which are adapted to be tuned each through a predetermined range of high frequencies, said winding being ooaxially wound on a common insulating tube and so spaced as to provide a certain degree of coupling between the windings, a first axially-adjustable core of comminuted magnetic material disposed within the tube and in overlapping relation with the adjacent end portions of said windings for increasing the degree of coupling between the windings and, upon adjustment thereof, for altering the inductance of the secondary without substantially effecting a change in the increased degree of coupling between the windings, and a second axially-adjustable core of comminuted magnetic material disposed within the tube and cooperatively related with the primary winding for altering the inductance of said latter winding.

3. A wide-band high-frequency transformer comprising a pair of windings which are adapted to'be tuned each through a predetermined range of high frequencies, said windings being of different lengths, coaxially wound on a common insulating tube and so spaced as to provide a certain degree of coupling between the windings, a

first axially-adjustable powdered-iron-core disposed within the tube in overlapping relation with the adjacent end portions of said windings for increasing the degree of coupling between the windings and, upon adjustment thereof, for altering the inductance of the shorter winding without substantially effecting a change in the increased degree of coupling between the windings, and a second axially-adjustable powerediron-core shorter than the first core i posed within the tube and cooperatively related with the longer winding for altering the inductance of said latter winding.

4. A wide-band high-frequency transformer comprising a pair of windings which are adapted to be tuned each through a predetermined range of high frequencies, said windings being coaxially wound on a common insulating tube andso spaced as to provide a predetermined degree of coupling between them. a pair of form-magnetic cores axially adjustable within said tube adapted each 6 upon adjustment to vary the self-inductance of one of said windings, one of said cores .being disposed in the field of only one of the windings, and the other of said cores being disposed in overlapping relation with the adjacent ends of both windings so that for all adjustments the latter core is in the magnetic fields of both windings to thereby increase the coeflicient of coupling between said windings to a maximum, said latter core being sufnciently longer than the spacing between the adjacent ends of the windings and sufilciently shorter than the spacing between the remote ends of the windings, that. the axial movement required to produce the desired adjustment in self-inductance of the winding other than said one winding results in no appreciable change in coupling between the two windings.

5. A wide-band high frequency transformer in accordance with the invention defined in claim 4 wherein the second core which serves to increase the coupling between the windings is of a different permeability material than the first core.

6. A wide-bang high-frequency transformer for a radio frequency amplifier or for an intermediate frequency amplifier of a superheterodyne receiving system, comprising primary and secondary windings coaxially wound on a common insulating tube and so spaced axially as to provide a certain degree of magnetic coupling between the windings, an adjustable ferro-magnetic core disposed within the tube and of such length that for all adjustments it overlaps the adjacent ends of said windings thereby increasing the degree of coupling therebetween, said core upon axial adjustment adapted to vary the inductance of one of the windings without affecting the degree of coupling between said windings, said core being of greater length than said one winding, and a second farm-magnetic core movable within the. other winding adapted upon adjustment thereof to vary the inductance of said latter winding, said second core being of shorter length than said other winding.

' 7. A wide-band high-frequency transformer in accordance with the invention defined in claim 6 wherein the second core is associated with the primary winding.

8. A wide-band high-frequency transformer in accordance with the invention defined in claim 'I wherein the second core is associated with the secondary winding.

9. A wide-band high-frequency transformer in accordance with the invention defined in claim '7 wherein the windings are of unequal axial lengths and the cores are of unequal axial lengths, the shorter core being associated with the longer winding and the longer core serving as the cou- J pling core.

DONALD MACKEY.

nnrsnnncns man The following references are of record in the 

